The present invention relates to a process for producing food grade alkali metal phosphates from wet process phosphoric acid. The particular alkali metal phosphates encompassed by the present invention are monosodium phosphate (NaH.sub.2 PO.sub.4), disodium phosphate (Na.sub.2 HPO.sub.4), trisodium phosphate (Na.sub.3 PO.sub.4) and mixtures thereof; monopotassium phosphate (KH.sub.2 PO.sub.4), dipotassium phosphate (K.sub.2 HPO.sub.4), tripotassium phosphate (K.sub.3 PO.sub.4) and mixtures thereof; and monoammonium phosphate (NH.sub.4 H.sub.2 PO.sub.4), diammonium phosphate [(NH.sub.4).sub.2 HPO.sub.4 ] and mixtures thereof. These alkali metal phosphates, also referred to as orthophosphates, find considerable use in food and related industries wherein their purity must adhere to stringent requirements.
The Food Chemicals Codex, Second Edition, published by the National Academy of Science in Washington, D.C., 1972, has compiled a list of standards for Food Grade Chemicals that has been given official recognition by the Food & Drug Administration.
The limits of impurities for the food grade alkali metal phosphates are listed in the table below:
______________________________________ Maximum Impurity Limits.sup.1 Arsenic Fluoride Heavy Metals Insolubles ______________________________________ (as Pb) NaH.sub.2 PO.sub.4 3 50 10 0.2% Na.sub.2 HPO.sub.4 3 50 10 0.2% Na.sub.3 PO.sub.4 3 50 10 0.2% KH.sub.2 PO.sub.4 3 10 20 0.2% K.sub.2 HPO.sub.4 3 10 20 0.2% K.sub.3 PO.sub.4 3 10 20 0.2% NH.sub.4 H.sub.2 PO.sub.4 3 10 10 -- (NH.sub.4).sub.2 HPO.sub.4 3 10 10 -- ______________________________________ .sup.1 Parts per million (ppm), unless otherwise specified.
In general, the primary method for producing food grade alkali metal phosphates occurs through a route beginning with the production of phosphoric acid via the "electric furnace" or "thermal acid" method well known in the art, and described in detail in Slack, Phosphoric Acid, Vol. 1, pages 927-966, Marcel Dekker, Inc. (1968), and Waggaman, Phosphoric Acid, Phosphates and Phosphatic Ferilizers, pages 158-173, Hafner Publishing Co. (2nd Edition, 1969).
The mono- and disodium phosphates are usually made from phosphoric acid and soda ash. In some plants soda ash is continuously mixed with the acid in such proportions as to keep the composition at that corresponding to disodium phosphate. Part of the resulting slurry is then cut back with acid to form monosodium phosphate. Trisodium phosphate is made by adding caustic soda to the disodium phosphate slurry. The trisodium composition cannot be reached with soda ash alone, and some caustic (about two-thirds, or more, of one equivalent or two-ninths, or more, of the total sodium) must be used. Conversion to sodium polyphosphates is generally accomplished using the sodium orthophosphates as a starting point, and is well known in the art.
The manufacture of food grade alkali metal phosphates wherein the thermal method for producing phosphoric acid is utilized, requires relatively expensive capital equipment, large amounts of electrical energy and pollution control. Because of the wide gap between the price of wet process phosphoric acid and the price of electrothermal acid made from elemental phosphorus, production of food grade alkali metal phosphates from wet process phosphoric acid has received increasing attention.
The production of "wet process" phosphoric acid by the treatment of phosphate rock with sulfuric acid is well known in the art and is described, for example, in Waggaman, Phosphoric Acid, Phosphates and Phosphatic Fertilizers, pages 174-209, Hafner Publishing Company (2nd Edition, 1969). The production of wet process phosphoric acid by the treatment of phosphate rock with acids other than sulfuric acid, such as hydrochloric acid and nitric acid is also well known in the art and is described in Slack, Phosphoric Acid, Volume 1, Part 2, pages 889-926, Marcel Dekker, Inc. (1968).
Regardless of what acid is used to produce the wet process phosphoric acid, a considerable amount of dissolved and suspended impurities such as silica, aluminum, magnesium, vanadium, fluorine, chlorine, calcium, arsenic, lead, iron and other organic and inorganic impurities are contained in the acid. The resulting wet process phosphoric acid contaminated by the aforesaid impurities has relatively limited use, primarily in the fertilizer industry where such impurities are not only not harmful but may be considered a desirable source of trace elements.
The prior art does contain methods for purifying wet process phosphoric acid, however, these methods are not entirely satisfactory, primarily due to the fact that they do not produce phosphoric acid as pure as that produced by the well known thermal process. Alkali metal phosphates produced from this acid would, therefore, retain these impurities. In addition, these prior art processes are costly and tedious, and, because of certain steps such as centrifugation, distillation, etc., which require large amounts of energy and costly equipment, these processes are not economical.
It has, therefore, been an objective of the industry to provide an efficient, economical method for purifying wet process phosphoric acid in order to obtain a product that can be used to produce food grade alkali metal phosphates.
The present invention has achieved an efficient and commercially economical method for producing food grade alkali metal phosphates from wet process phosphoric acid. It is characterized by high yields, it is not energy dependent and is simple in operation. In addition, all materials used can generally be reused after conditioning.